The invention relates to a method for the cryogenic separation of air, in which compressed and purified application air is cooled in a main heat exchanger and is supplied at least in part to a rectifying column, a first partial flow of the application air being removed from the main heat exchanger at an intermediate temperature and being supplied to a cold compression at this intermediate temperature.
A method and a device for the cryogenic separation of air are known, for example, from xe2x80x9cTieftemperaturtechnikxe2x80x9d, 2nd Edition, 1985, Chapter 4 (Pages 281 to 337) by Hausen/Linde.
The invention is used in those cases in which a portion of the application air (xe2x80x9cfirst partial flowxe2x80x9d) is aftercompressed, for example, in order to be used for the evaporation of a liquid process flow. The liquid process flow may be a product flow (such as liquid oxygen, liquid nitrogen or liquid argon) from a rectifying column; the sump liquid or intermediate liquid of a rectifying column; or an external liquid which is taken, for example, from a storage tank. It is also possible to evaporate two or more such process flows against the aftercompressed partial air flow.
The xe2x80x9cmain heat exchangerxe2x80x9d is preferably formed by a single heat exchanger block. In the case of larger systems, it may be useful to implement the main heat exchanger by several pipe trains which are connected in parallel with respect to the temperature course and which are formed by mutually separate structural elements. In principle, it is also conceivable that the main heat exchanger or each of these pipe trains is formed by two or more serially connected blocks.
In many cases, this aftercompression is carried out in a conventional manner in that the partial air flow is supplied to a corresponding machine approximately at ambient temperature. As an alternative, a cold compressor can be used for the aftercompression. In this case, xe2x80x9ccold compressionxe2x80x9d is a compressing operation in which the gas is fed to the compression at a temperature which is clearly below the ambient temperature, generally below 250 K, preferably below 200 K.
Methods are known from International Patent Document WO 9528610 or European Patent Document EP 644388A, in which the cold compression is carried out at an intermediate temperature which is between the temperatures at the warm and cold end of the main heat exchanger. This intermediate temperature may particularly be at the point at which the curves of the flows to be warmed up and to be cooled come closest to one another in the heat exchange diagram (Q-T diagram) of the main heat exchanger (xe2x80x9ctheoretical pinch pointxe2x80x9d).
In the known methods, the partial air flow, which leads to the cold compression, is cooled in the main heat exchanger from the warm end to the intermediate temperature and, at the corresponding intermediate point of the main heat exchanger, is taken out directly from the cooling passages.
It is an object of the invention to provide the method of the initially mentioned type and a corresponding device which, with respect to energy, can be operated particularly advantageously.
This object is achieved in that the first partial flow is warmed up upstream of its removal in the main heat exchanger.
According to the invention, the partial air flow provided for the cold compression is therefore first cooled more than actually necessary in the main heat exchanger, thus beyond the intermediate temperature which corresponds approximately to the inlet temperature of the cold compression. Subsequently, it is warmed up againxe2x80x94also in the main heat exchangerxe2x80x94to the intermediate temperature. At first glance, this method of operation seems disadvantageous because, as a result of the cooling and reheating, which is unnecessary per se, additional exchange losses and therefore a higher energy consumption are to be expected. However, within the scope of the invention, it was found that, as a result, the heat transfer is improved in the cold part of the main heat exchanger (below the intermediate temperature).
The reason is that, in the cold part of the main heat exchanger, the flows to be warmed up and cooled off have a higher density than in the warm part. The heat exchanger passages, through which they flow, for constructive reasons, as a rule, have the same number and the same cross-sections. In the cold part, the passages are, as it were, operated with an underload of approximately 20%. Because of this fact, the flow conditions are not optimal in the cold part of the main heat exchanger. The invention achieves an improvement here, in that the partial air flow for the cold compressionxe2x80x94which has to be subjected to a special treatment anyhowxe2x80x94supplements the flows which are to be cooled as well as the flows which are to be warmed up. It was found that the improvement of the heat transfer as a result of the flow conditions optimized within the scope of the invention in the cold part of the main heat exchanger overcompensates the expected additional exchange losses and, on the whole, results in a process which is particularly favorable with respect to energy. Also, the additional mass flow in the cold part of the main heat exchanger results in a steeper course of the curves of the flows to be warmed up and cooled down in the Q-T diagram and thus in an improvement at the point where these curves comes closest to one another (xe2x80x9ctheoretical pinch pointxe2x80x9d).
The first partial flow can be at least partially liquified downstream of the cold compression against an evaporating process flow. This heat exchange step can be carried out either in the main heat exchanger or in a separate condenser evaporator This method of operation will be particularly advantageous if the entire oxygen product or a large portion thereof is removed from the rectification as a liquid, is pressurized in liquid form and is finally evaporated against the cold-compressed partial air flow. In this case, just as much air is cold-compressed to ensure that the flow conditions in the cold part of the main heat exchanger are virtually optimal as a result of the reheating of this partial air flow according to the invention.
Preferably, the first partial flow is introduced into the cold end of the main heat exchanger before its warm-up. It is therefore first guided completely through the main heat exchanger and, when being warmed up, flows again through the entire cold part of the main heat exchanger, so that the entire cold part of the main heat exchanger benefits from the improved flow-through.
In this case, the cooling of the first partial flow can be carried out separately from or jointly with other portions of the application air For this purpose, a cooling air flow is cooled in the main heat exchanger, is taken out at the cold end of the main heat exchanger, and, at least partially, is fed again as a first partial flow to the cold end of the main heat exchanger.
In the case of the method according to the invention, it may be advantageous to separate liquid fractions before the rewarming of the first partial flow. For this purpose, after having been taken out of the cold end of the main heat exchanger, the cooling air flow is subjected to a phase separation, during which the first partial flow is formed at least by one part of the vapor phase taken out of the phase separation. Preferably, the entire vapor fraction from the phase separation is led to the cold compression, while the separated liquid is fed into the rectifying column or one of the rectifying columns, for example, into the pressure column of a two-column apparatus
Particularly in this case, it is advantageous for the cooling air flow to be expanded before it is subjected to the phase separation. However, also when a phase separation is absent, it may be useful to throttle off the cooling air flow before it is fed as a first partial flow to the cold end of the main heat exchanger.
In principle, the entire flow subjected to the cold compression can be formed by the first partial flow which is withdrawn from the main heat exchanger at the intermediate point. However, in many cases, it is more advantageous to divide the cooling air flow into a first partial flow and into a second partial flow, the first partial flow being introduced into the cold end of the main heat exchanger, and the second partial flow, without temperature changing measures, together with the first partial flow, being fed between its withdrawal at the first intermediate temperature and the cold compression. As a result, cold temperature is additionally introduced into the cold compression flow and is used for the partial or complete compensation or perhaps even overcompensation of the compression heat generated during the cold compression. As a result, an additional parameter is obtained which can be used for optimizing the heat exchange process.
The first partial flow can be fed to the cooling air flow downstream of the cold compression at an intermediate point of the main heat exchanger which corresponds to a second intermediate temperature. Without the compensation of the compression heat described in the previous paragraph, this second intermediate temperature is above the first intermediate temperature. When being mixed with the very cold second partial air flow upstream of the cold compression, the second intermediate temperature may be at or even below the first intermediate temperature.
In addition, it is advantageous for a turbine air flow in the main heat exchanger to be cooled to a third intermediate temperature and to be subsequently expanded in a work-performing manner, in which case at least a portion of the mechanical energy generated during the work-performing expansion is used for driving the cold compression. If the cold temperature required for the process is not generated by an additional expansion machine, it is necessary to couple the expansion machine not only with the cold compressor but, in addition, with a generator or a brake fan.
The invention also relates to a device for the cryogenic separation of air. For example, in one embodiment, the invention includes a device for the cryogenic separation of air having a main heat exchanger which has a warm and a cold end as well as groups of cool-down and warm-up passages, having at least one rectifying column, having an application air line for feeding compressed and purified application air to the main heat exchanger and for feeding at least a portion of the cooled application air into the rectifying column, and having a cold compression line which extends from an intermediate point of the main heat exchanger to a cold compressor, characterized in that the cold compression line is connected upstream of the cold compressor at the intermediate point with a group of warm-up passages of the main heat exchanger. In another embodiment, the invention is further characterized in that the group of warm-up passages of the main heat exchanger, which are connected at the intermediate point with the cold compression line, have a continuous construction from the cold end to the intermediate point and are connected at the cold end with a group of cool-down passages.